The present invention relates to an electromagnetic valve provided with two opposed valve seats and three ports.
A two-seat electromagnetic valve is known, which is constructed basically in the following manner. That is, a housing is formed therein with a valve chamber defined between a pair of opposed faces. A pair of valve bores are formed respectively in the pair of opposed faces. The valve chamber is connected to an actuator through a control port. One of the pair of valve bores is connected to a pump through a supply port, while the other valve bore is connected to a tank through a discharge port. A valve member is accommodated in the valve chamber. When the valve member is moved away from a first valve seat formed about the one valve bore and is abutted against a second valve seat formed about the other valve bore, the control port is brought into communication with the supply port, but is intercepted from communication with the discharge port. On the other hand, when the valve member is abutted against the first valve seat, the control port is intercepted from communication with the supply port, but is brought into communication with the discharge port. Movement of the valve member is controlled by a coil spring arranged within the housing and a solenoid drive unit for generating force opposite to biasing force of the coil spring.
The known two-seat electromagnetic valves include ones of so-called ball type, flat type and cone type. The two-seat electromagnetic valve of ball type employs a valve member in the form of a ball, as disclosed in Japanese Patent Publication No. 49-10371 and Japanese Patent Application Laid-Open No. 60-53289. In the two-seat electromagnetic valve of flat type, each of a pair of valve seats is formed by a planar or flat face extending perpendicularly to an axis of a valve member and, likewise, each of a pair of abutment faces of the valve member capable of being in contact respectively with the valve seats is also formed by a planar or flat face extending perpendicularly to the axis of the valve member, as disclosed in Japanese Utility Model Application Laid-Open No. 61-140297 and the corresponding U.K. Pat. No. 2,173,550. The two-seat electromagnetic valve of cone type employs a valve member having a pair of abutment faces each of which is formed by a conical face, as disclosed in Japanese Patent Application Laid-Open No. 60-44671.
The above-described electromagnetic valves of ball type and cone type are widely used in pressure control on the basis of duty control. Specifically, when the solenoid drive unit is supplied with drive current in the form of pulses duty-controlled, a cross-sectional passage area between each of the pair of valve seats and a corresponding one of the pair of abutment faces of the valve member is determined correspondingly to the duty ratio of the drive current, thereby controlling hydraulic fluid pressure supplied to the actuator which is connected to the control port.
The two-seat electromagnetic valve of cone type disclosed in the above Japanese patents is in particular superior in response. This is because the opposed axial forces acting on the valve member due to application of the hydraulic fluid pressure to the valve member are canceled out with each other so that the valve member is moved under the biasing force of the coil spring and the force of the solenoid drive unit, substantially regardless of the pressure of the hydraulic fluid. Thus, the electromagnetic valve of cone type is suitable for use in relatively high-frequency duty control.
The pressure control is carried out on the basis of the duty control. Flow of the hydraulic fluid is restricted extremely, in the state in which the cross-sectional passage area between either one of the valve seats and a corresponding one of the abutment faces of the valve member is small. By this reason, the pressure at the control port do not vary in linear relation to displacement of the valve member and, accordingly, the pressure at the control port do not also vary in linear relation to the duty ratio.
It is required for the two-seat electromagnetic valve of cone type to displace the valve member relatively largely from the position where the valve member is seated on either one of the valve seats, in order to exceed the cross-sectional passage area which does not produce the above-mentioned linear relationship. The reason for this is that, since the abutment face of the valve member is inclined with respect to the axis of the valve member, variation of the cross-sectional passage area is small with respect to an amount of displacement of the valve member.
Thus, it is impossible for the two-seat electromagnetic valve of cone type to control the pressure on the basis of the duty control within a relatively wide range of from 0% to a certain lower value of the duty ratio and within a relatively wide range of from a certain high value to 100% of the duty ratio. In other words, a dynamic range defined between the certain lower and upper values, which is capable of controlling the pressure at the control port linearly, is narrow in width. By this reason, pressure variation at the control port cannot but be brought to a large value with respect to a given variation of the duty ratio. As a result, it has been difficult to carry out the pressure control with high accuracy or precision.